Epoxy resin-based fluxing underfill adhesives are widely used for the assembly of solder-bumped flip-chip devices. As these assemblies are heated in the region of 180-220° C. to reflow or melt the solder, the fluxing agent component of the adhesive cleans and removes oxides from solder surface promoting self-alignment, electrical connectivity and mechanical attachment of the chip to the substrate. During this process, the bumped chip is observed to undergo a controlled collapse as solder wets the contact pads located on the substrate surface and a solder joint is formed. To facilitate this process, it is important that curing of the epoxy be kept to a minimum until device collapse has occurred. Thereafter, it is desirable that curing be completed as quickly as possible.
Current epoxy formulations have been optimized for this process which generally employs lead-based solder alloys. However, due to toxicity and environmental concerns, no-lead solders, based on silver alloys, have been developed. These alloys typically melt at temperatures that are significantly higher than lead-based materials (i.e., 210-250° C.) and therefore fluxing formulations designed for lead-based processes are ineffective with silver-based alloys. The higher temperatures of the lead-free alloys cause significant curing of the adhesive compositions prior to and during fluxing and solder melting. As a result, device collapse and electrical connectivity are seriously compromised.
A need exists therefore, for fluxing adhesive formulations in which the onset of curing occurs at substantially higher temperatures than is possible with current epoxy systems, but yet retains other desirable features of the existing formulations.